This invention relates to cleaning of stack gases such as those from coal fired power plants, from natural or propane burning heating plants or from cement kilns. The stack gases exhausted from each such facility is controlled by environmental regulations. Such regulations require abatement of carbon monoxide (CO), carbon dioxide (CO2), nitrogen oxide (NOx), sulfur oxide (SOx) as well as halogens such as chloride and fluorides and trace metals particularly, mercury, lead, zinc.
Various methods and apparatus have been proposed for abating these pollutants in stack gases. It is proposed that the stack gases be mixed with ammonia or urea and then passed through a catalyst in which the ammonia reacts selectively with the nitrous oxides to form nitrogen gas in water vapor, or combustion of a sulfur-containing fossil fuel in the presence of a calcium carbonate or magnesium carbonate to form calcium sulfate or magnesium sulfate. See U.S. Pat. Nos. 8,181,451, 7,968,068, 6,706,246, 5,525,317, 5,237,939, 4,185,080, and 4,051,225. Reducing nitrogen in stack gas passing the stack gas through a heat exchange having a SCR catalyst. See U.S. Pat. No. 5,918,555. Reduction of sulfur oxide content in stack gases by catalyzed oxidation to sulfur trioxide in the presence of an absorbent or combusting sulfur-containing fuel in a combustion zone charged with a slurry in sulfuric acid solution. See U.S. Pat. Nos. 5,540,755, 4,649,034, 4,284,015, and 4,185,080. Catalytically converting unburned hydrocarbons and carbon monoxide to carbon dioxide and reducing nitrogen oxides to nitrogen subsequent to the combustion of fossil fuels while absorbing sulfur oxide, where the catalytic material is physically combined onto a dry powder of an adsorbent matrix select from calcium aluminate, calcium aluminate cement, barium titanate, and calcium titanate. See U.S. Pat. No. 4,483,259. It has also been proposed to pass the stack gases through a catalyst bed of a combination of active metals on the surface that is capable of reducing or converting sulfur oxides, carbon monoxide and hydra carbons to inert compounds such as carbon dioxide, water and nitrogen. See U.S. Pat. No. 7,399,458. Levels of mercury in stack gases from coal combustion have also been reduced by introducing a sorbent composition into the gas stream in a zone where temperature is greater than 500° C., where the sorbent composition comprises an effective amount of nitrate salt and/or a nitrite salt. See U.S. Pat. Nos. 7,468,170 and 7,731,781.
These previous proposals had a number of drawbacks. Many require addition of another gas or liquid such as ammonia or sulfuric acid, or the presence of an active metal catalyst. Also, they have involved production of stack gases such as carbon dioxide, which is now itself regulated as pollutant, and production of a waste material which itself presents a disposal problem sometimes as a hazardous waste.
There is still a need for a method and apparatus to effectively remove carbon monoxide, carbon dioxide, nitrous oxides, and sulfur oxides, as well as trace metals such as mercury, from stack gases without consuming expensive catalysts, injection of additional gases and solids, and without creating waste products that do, themselves, present the problems in disposal.
This problem in cleaning of stack gases is of particular concern in coal fire power plants because of the release of volatiles such as coal tar, sulfur oxides, and other active pollutants. These proposals also involve flaring methane gas which could reduced with energy recovery. A variety of methods have been proposed for reducing pollutants released from coal fired power plants. One method of cleaning coal flue gases is the use of scrubbers that inject a liquid or slurry into a gas stream that washes various pollutants, such as acidic compounds, from the flue stream. Another type of cleaning is the use of an exhaust burner that combusts volatile materials and other compounds to reduce pollution. The burner may also burn uncombusted methane present in the exhaust stream. Another type of cleaning that may be used is carbon capture that collects and stores carbon dioxide, such as by compressing the carbon dioxide and storing it in a geological formation. Other types of cleaning flue gases from coal fired plant have also been proposed and are known to those having skill in the art.
Zeolite has also been proposed as a material to absorb carbon dioxide, then to be able to regenerate the zeolite material. See “Carbon Dioxide Capture Using a Zeolite Molecular Seive Sampling System for Isotopic Studies (13C abd 14C) of Respiration,” Radiocarbon, 47, 441-451 (2005), “Absorbent Materials for Carbon Dioxide Capture from Large Anthropogenic Point Sources, ChemSusChem 2009,2, 796-854, NIST Provides Octagonal Window of Opportunity for Carbon Capture, NIST Techbeat, Feb. 7, 2012. However, these uses of zeolite involved large particle sizes of zeolite, for example between 1/32 and 1/16 inch in size in a low pressure to provide for adsorption of carbon dioxide and later regeneration. The effectiveness of these processes have been affected by the presence of moisture.
The presently disclosed method uses more finely ground zeolite as described below to more effectively clean carbon oxides, sulfur oxides and nitrogen oxides from stack gases. The method of cleaning stack gases disclosed comprises the steps of: (a) providing in a stack or flue adapted to pass stack gases a first catalytic flow-through bed of calcium zeolite comprising zeolite particles of a majority between 44 μm and 64 μm in size adapted to reduce carbon oxides from the stack gases, (b) providing in the stack adapted to pass stack gases positioned adjacent the first catalytic flow-through bed, a second catalytic flow-through bed of a blend between 25 and 75% of sodium zeolite and calcium zeolite comprising zeolite particles of a majority between 65 μm and 125 μm in size adapted to reduce nitrogen oxides from the stack gases, (c) providing in the stack adapted to pass stack gas positioned adjacent the second catalytic flow-through bed, a third catalytic flow-through bed of calcium zeolite comprising zeolite particles of a majority between 78 μm and 204 μm adapted to reduce sulfur oxides in the stack gases The method further include passing stack gases selected from the group consisting of volatiles from combustion of coal or from combustion of natural gas or propane or flue gases from a cement kiln sequential through the first catalytic bed, the second catalytic bed, and the third catalytic bed each reducing pollutants and collecting solids in the catalytic beds and providing gas exiting the third catalytic bed with at least 90% reduction in sulfur oxides, nitrogen oxides, and carbon oxide. The particle size ranges of calcium zeolite and the calcium zeolite/sodium zeolite blend is indicative of the surface area, number, of exposed zeolite pores, and ion exchange capacity of the calcium and sodium/calcium zeolite in the catalytic flow-through beds.
The first catalytic flow-through bed, the second catalytic flow-through bed and the third catalytic flow-through bed may be provided between screens of between 150 and 350 mesh. This enables the catalytic beds to contained in position without falling out of the bed, while maintaining the catalytic beds as flow through beds without appropriate pressure drop across each bed. The beds can be maintained in any desired cross section given cross-sectional area of the stack and with a depth corresponding to the flow rate of stack gases through the stack.
The method can be extended by purging solids and liquids from the first catalytic bed, the second catalytic bed, and the third catalytic bed by intermittently passing nitrogen through the beds to remove solids and liquids reaction product formed in the beds from the stack gases. This embodiment enables the solids and liquids collected in the catalytic flow-through beds to be reacted with nitrogen to form nitrate compounds and be recovered as a useable product such as fertilizer. This step of purging the catalytic flow-through beds may be performed by liquid nitrogen to increase the efficiently of the recovery of solids and liquids from the catalytic flow-through beds and make the recovered product more useful as a fertilizer product.
This method also provides a way of removing other such as aluminum oxides, mercury compounds and trace metals such as zinc, lead, uranium and rare earth metals from the stack gas stream. An added benefit of the present method is that methane, which is typically flared and wasted, can be recovered, and moisture typically a problem is reduced as part of the recovered product and taken off as hydrogen and oxygen gas in the cleaned stack gas.
Furthermore, the first catalytic flow-through bed, the second catalytic flow-through bed, and the third catalytic flow-through bed can be provided in a system that allows for continuous reduction and cleaning of stack gas while another part of the same beds or like beds are purged to capture of fertilizer products and other commercial products and clean the beds for reuse. For example, two or more series of beds could be provided in parallel so one series of beds were in use to clean the stack gas while one or more other series of beds was be purged of collected solids and liquids and cleansed for reuses. Another approach is to assemble the beds on rotating disks. This enables the catalytic flow-through beds to be maintained in operation cleaning the stack gases while other parts the same catalytic flow-through bed or a bed of similar composition to be cleaned of collected solids and liquids for reuse. The later approach has the advance of allowing the individual beds to be taken out of use separately as desired, rather than require all beds in a series to be taken off line at once for purging to removing collected solids and liquid and clean for reuse.
In another embodiment, an additional fourth catalytic flow-through bed of calcium zeolite comprising zeolite particles between 44 μm and 64 μm in size may be positioned in the stack gas before the first catalytic bed, with an electrical charge beneath said fourth catalytic flow-through bed to facilitate collection aluminum oxides and other aluminum compounds from from the stack gases before passing through the first catalytic bed. This embodiment enables aluminum compounds to be separately capture and recovered as a separate product that can be a substitute for bauxite to produce aluminum, particularly where the beds are provided on rotating disks so the beds can separately taken off line for removal of collected solids and liquids. This is particularly useful with some coals high in aluminum oxides and given bauxite ore is not readily available in some location. If the aluminum compounds are not separately recovered by recovered in the first catalytic flow-through bed the aluminum compounds go into the fertilizer products recovered using this method.
Irrespective of the embodiment of the present method and apparatus that is employed at least 95% or 99% reduction in sulfur oxides, nitrogen oxides and carbon oxides compared to the content in the stack gases delivered to the a first catalytic flow-through bed. The present method can also be useful for focusing on collection of sulfur oxides where sulfur oxides are particularly a problem such as in coal fired plants of 50 to 100 KW. These plants are targeted by EPA for conversion to natural gas or propane fired plants at considerable cost. The present method allow these plants to continue to operate as coal fired power plants and meet the EPA sulfur oxide standards for sulfur emission. The method also results coal-fired power plants of less the 50 KW to continue to operate, where these power plant would otherwise are targeted to shutdown because of inability to comply with new EPA sulfur oxide standards.
Also disclosed is apparatus of cleaning carbon oxides, sulfur oxides, nitrogen oxides from stack gases comprising: (a) assembling an stack adapted to pass stack gases selected from the group consisting of volatiles from combustion of coal or from combustion of natural gas or from flue gas from cement kiln, (b) assembling in the stack adapted to pass stack gases a first catalytic flow-through bed of calcium zeolite comprising zeolite particles of a majority between 44 μm and 64 μm in size adapted to reduce carbon oxides from the stack gases, (c) assembling in the stack adapted to pass stack gases positioned adjacent the first catalytic flow-through bed, a second catalytic flow-through bed of a blend between 25 and 75% of sodium zeolite and calcium zeolite comprising zeolite particles of a majority between 65 μm and 125 μm in size adapted to reduce nitrogen oxides from the stack gases, (d) assembling in the stack adapted to pass stack gas positioned adjacent the second catalytic flow-through bed, a third catalytic flow-through bed of calcium zeolite comprising zeolite particles of a majority between 78 μm and 204 μm adapted to reduce sulfur oxides in the stack gases. The method results in the stack gases flowing through the first catalytic bed, the second catalytic bed, and the third catalytic bed adapted to collect solids in the catalytic beds so that stack gases exiting the third catalytic have at least 90% reduction in sulfur oxides, nitrogen oxides and carbon oxide. This apparatus may be utilized to provide at least 95% or 99% cleaning of the stack gas.
The first catalytic flow-through bed, the second catalytic flow-through bed and the third catalytic flow-through bed may each be assembled between screens of between 150 and 250 mesh to provide for flow-through while maintaining the zeolite particles in the beds. The beds can be accommodate any desired cross section according the cross-sectional area of the stack, with a depth corresponding to the flow rate of stack gases through the stack.
The first catalytic flow-through bed, the second catalytic flow-through bed, and the third catalytic flow-through bed can be assemble with parallel series of the first catalytic flow-through bed, the second catalytic flow-through bed and the third catalytic flow-through bed in a system that allows for continuous reduction and cleaning of stack gas while one or more series of like beds are purged to capture of fertilizer products and other commercial products and clean the beds for reuse. Another approach is to assemble the beds on rotating disks. This enables the catalytic flow-through beds to be maintained in operation cleaning the stack gases while other parts the same catalytic flow-through bed or a bed of similar composition is cleaned of collected solids and liquids for reuse. The later approach has the advance of allowing the individual beds to be taken out of use separately as desired, rather than require all beds in a series to be taken off line at once for purging to removing collected solids and liquid and clean for reuse. The first catalytic bed, the second catalytic bed, and the third catalytic bed may each be assembled on a rotating disk so a part of the bed or a similar bed can be maintained to continue to clean stack gas without interruption, while another portion of the bed or a similar bed is cleaned for reuse.
Also disclosed is the fertilizer product made by the processes described in the previous application. This product may include trace metals as such zinc compounds, lead compounds mercury compounds, uranium compounds, and rare earth metal compounds, as well as sulfate and fluoride compounds beneficial in fertilizers.
Other details, objects and advantages of the present invention will become apparent from the description of the preferred embodiments described below in reference to the accompanying drawings.